ROCK TUNNELS | DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE
Compressible material
EXC.
Concrete lining
Right, figure 3:
Schemes of main phases of a gallery to be sealed
(1) EXCAVATION CREEP R.H. SEAL
NAT. HYD.
(3) OPERATIONAL PHASE (4) POST-CLOSURE PHASE (2) LINING CONSTRUCTION
For the numerical simulation, it is important
to identify and understand all the processes and phenomena involved behind the response of the sealing structures that may affect their performance. At this stage, the uncertainties related principally to: how the self-sealing contribution should be considered in performance assessments; and, how sealing structures will be able to reproduce, by means of swelling pressure, the mechanical effect that helps to restore low permeability of the COx claystone. This paper looks at the latter aspect. The sealing structure is intended to be placed
relatively far from the storage zone and, therefore, from heat sources. For this reason, isothermal conditions are assumed and long-term effects of temperature are not considered. Physico-chemical processes between bentonite and the nearby materials, and gas pressurisation due to degradation and corrosion of the support elements, are not considered. Under these considerations, there are four main
phases in the life of a gallery and its sealing, shown in Figure. 3 and summarised as follows: ● Phase 1 - excavation procedure, creating the EDZ; ● Phase 2 - excavation lining. Consists of two layers: primary lining (50cm thickness) is concrete;
secondary (20cm) is compressible, made from reformed COx claystone.
● Phase 3 - facility operation for 100 years. Galleries are open access tunnels. Must consider interaction of the COx claystone and the relative humidity caused by tunnel ventilation. Interaction between the compressible secondary lining and the host rock also plays an important role, due to long-term behaviour associated with rock creep deformations and hydro- mechanical evolution towards equilibrium.
● Phase 4 - post-closure, when the most important phenomena take place (see Figure. 4).
Before seal construction, the draft lining will be partially removed, by portions, over the length of the core, to ensure direct contact between the expansive core and the host rock at certain points. During the post-closure phase, water seeps from the
host rock towards the sealing core. The core begins to swell, exerting pressure on the host rock and concrete plugs, which slide and compress the drift backfill, releasing some of the swelling pressure in the core. Equilibrium is reached when the expansive core pressure (longitudinal, parallel to the drift) is countered by the stabilising forces. Sealing performance is strongly related to the stability of the entire structure.
NATURAL HYDRATION
Right, figure 4: Scheme of main phenomena during post-closure phase
Disp. SWELLING strength Shear
Backfill
Plug
Core
22 | February 2025
EDZ
Reaction
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